Cat tested at 7 normal is 4 for thyroid. (Sorry, I don't know what this test was, just referred to as 'thyroid level' by my vet.) All other blood work fine (kidnew, liver, blood sugar etc.) She is spayed 13 year old indoor cat. Was always ravenously hungry(also regular throwing up about 10 hrs after eating) prior to hyperthyroid diagnosis. Started Thurs afternoon on 5 mg methimazole twice a day. My vet is not in Fridays or Sat or Sundays except for expensive emergency care. By Friday morning cat has for the most part stopped eating and mopes like she doesn't feel well (hiding a lot), but is actually no longer throwing up as was her usual. Looks at food dish, won't touch anything all day. Does drink water, does use litterpan. Has eaten about 1/4 her normal amount during the overnight hours. Dosed at 7am and 7pm. I am wondering if perhaps vet made error and set does too high for size of cat? Does this dosage seem appropriate yes or no?

I'm not a Vet and I would never second guess a Vet BUT the dosage for adults (who presumably weigh more than your cat) is 5 or 10 mgs.

Seems like a high dosage for your cat. But, again, I didn't go to Vet school.

If it were my cat - and it's Sunday night "here" - I would discontinue the medication and call the Vet in the AM.

Methimazole comes in 5mg tablets generally and that is the starting dose for cats. I do not believe your vet made an error. It does come in 2.5mg tablets as well (or you can halve the 5mgs) for cats who test too low after 4 or so weeks of 5mg.
I've never seen a cat go home on more than 5mg initial dose, and if anything the symptoms would be overdosing than underdosing. However, it may be totally unrelated.
I would take your cat in tomorrow.
Edit: Now that I re-read that, our cats usually go home on 5mg once a day, I totally overlooked the twice a day.
It is possible kitty is getting too much. Hold off giving anymore, and go in tomorrow.